circularly linked list in python

Question

I am stuck implementing the add function of a circularly linked list in python. I have a head pointer that should be a reference to a node, but every time I add something to the list, head is always None. Here is the code I have so far:

class CircleList():

    __slots__ = ('head', 'size')

    def __init__(self):
        self.head = None
        self.size = 0

    def __str__(self):
        result = "<"
        node = self.head
        count = self.size
        while count != 0:
            result = result + str(node.data)
            if count != 1:
                result = result + ", "
            node = node.next
            count -= 1
        result = result + ">"
        return result

    def add(self, element):
        head = self.head
        print(head)
        size = self.size
        if head == None:
            head = Node(element, None)
            head.next = head
        else:
            cursor = head
            while cursor.next != head:
                cursor = cursor.next
            temp = Node(element, head)
            cursor.next = temp
        size += 1


class Node():
    __slots__ = ('data','next')

    def __init__(self, data, next):
        self.data = data
        self.next = next

Here is the driver:

stream = open('data.txt', 'r')

circlelist = CircleList()

for name in stream
    circlelist.add(name)

print(circlelist)

Any reason why you implement it like this? It's not exactly pythonic, and you could easily come up with a much more elegant version extending list, providing meta-methods like __getitem__ etc. and using indices. This also eliminates the need for the superfluous Node class. Also, I suggest sticking to the well-known interface of list. — Jim Brissom
– Jim Brissom, Commented Dec 5, 2010 at 17:23

Frédéric Hamidi · Accepted Answer · 2010-12-05 17:19:03Z

1

You only assigning the new node to your local head variable in your add() method, not to the actual CircleList instance member.

You might want to do something like:

def add(self, element):
    head = self.head
    print(head)
    size = self.size
    if head is None:
        self.head = head = Node(element, None)  # Also set the instance member.
        head.next = head

answered Dec 5, 2010 at 17:19

Frédéric Hamidi

264k42 gold badges497 silver badges486 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

salezica · Accepted Answer · 2010-12-05 17:15:32Z

0

Easy to fix! In your add function, you assign the new head to the head variable -- which is restricted to the scope of the function, and will dissapear when it returns!

You have to set the value of self.head, the attribute of the current instance.

Edit: when you assign head = self.head you are making them both point to the same object. But they are separate references: whether they happen to reference the same thing or not, changing one won't change the other.

answered Dec 5, 2010 at 17:15

salezica

77.5k29 gold badges113 silver badges168 bronze badges

Collectives™ on Stack Overflow

circularly linked list in python

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related